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Hierarchical FAU/ZIF-8 hybrid materials as highly efficient acid-base catalysts for aldol condensation Duangkamon Suttipat, Wannaruedee Wannapakdee, Thittaya Yutthalekha, Somlak Ittisanronnachai, Thasanaporn Ungpittagul, Khamphee Phomphrai, Sareeya Bureekaew, and Chularat Wattanakit ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00389 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Hierarchical FAU/ZIF-8 hybrid materials as highly efficient acid-base catalysts for aldol condensation Duangkamon Suttipat,† Wannaruedee Wannapakdee,† Thittaya Yutthalekha,† Somlak Ittisanronnachai, ‡ Thasanaporn Ungpittagul, ‡ Khamphee Phomphrai, ‡, Sareeya Bureekaew,† and Chularat Wattanakit*† †

Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering,

Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand ‡

Department of Materials Science and Engineering, School of Molecular Science and Engineering and

Frontier Research Center (FRC), Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand

KEYWORDS: Hierarchical zeolite, Metal-organic framework, Hierarchical FAU/ZIF-8 composites, Zeolite/MOF composite, Aldol condensation, 5-HMF, Zeolite/MOF composite

ABSTRACT. The composite of hierarchical faujasite nanosheets and zeolitic imidazolate framework-8 (Hie-FAU/ZIF-8) has been successfully prepared via a stepwise deposition of ZIF-8 on modified zeolite surfaces. Compared to the direct deposition of MOF on zeolite surfaces, ZIF-8 nanospheres were selectively attached to external surfaces of the MOF ligand-grafted FAU crystals because of the enhancing interaction between zeolite and MOF in the composite. In addition, the degree of surface functionalization can be greatly enhanced due to the presence of hierarchical structures. This behavior leads to increase in the deposited MOF content, improving the hydrophobic properties of zeolite surfaces. Interestingly, the designed hierarchical composite exhibits outstanding catalytic properties as an acid-base catalyst for the ACS Paragon Plus Environment

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aldol condensation of 5-hydroxymethylfurfural (5-HMF) with acetone. Compared to the isolated FAU and ZIF-8, the high yield of product, 4-[5-(hydroxymethyl)furan-2-yl]but-3-en-2-one (HMB) (67 %) can be observed in the composite due to the synergistic effect between Na+ stabilized zeolite framework and imidazolate linkers bearing basic nitrogen functions. This opens up interesting perspectives for the development of new organic and inorganic hybrid materials as heterogeneous acid-base catalysts.

1. INTRODUCTION The development of multifunctional hybrid porous materials is one of the most challenging tasks to design new compounds because of their outstanding unique properties, such as bifunctional acid-base functions, hydrophobicity, anti-poisoning, and anti-leaching properties.1-3 The most popular example of porous composites is the hybrid material composing of organic and inorganic moieties. 4-7 Considering the possible design of inorganic materials, zeolite is known to play an important role in the separation and catalysis technologies owing to their outstanding properties, including large surface areas, uniform and tunable micropores, tailorable chemical compositions, high adsorption capacities, tunable acidic-basic properties, high thermal/hydrothermal stabilities, high mechanical resistance and well-defined porous structures.8-10 However, there are some limitations when using such materials, in particular, the hydrophilicity of low Si/Al zeolite surfaces, eventually leading to the facile hydrolysis of zeolite framework.11 To circumvent this problem, various modification methods have been applied, such as the surface treatment with cationic metal ions,12-14 the grafting with organosilane molecules,15-16 and the surface deposition with carbon and silica residues 17-18. However, such modifications sometimes suffer from the reduction of the number of catalytic sites of zeolites and pore blocking due to incorporated grafting fragments.19-20 In addition, the low external surface area of a conventional zeolite eventually leads to the low possibility of surface functionalization degree. An alternative way to modify zeolite surfaces is the deposition of metal organic frameworks (MOFs) on their external surfaces. It is able not only to generate additional porous materials, but also to tune catalytic properties of zeolite surfaces.21-24 Recently, ACS Paragon Plus Environment


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Scheme 1. Illustration of fabrication steps of hierarchical FAU/ZIF-8 composites. Zhu et al. have successfully synthesized the zeolite@metal–organic framework (ZSM-5@UiO-66) with core–shell structures by the direct solvothermal growth of UiO-66 on external surfaces of ZSM-5 crystals.23 This material provides bifunctional active sites, including the Brønsted acid site of zeolite and the basic sites derived from the amine groups in UiO-66 and shows very promising performances for the cascade reaction of Knoevenagel condensation. In addition, Gao et al. have successfully synthesized the core-shell 5A@ZIF-8 composites by using a combined method of a pre-seeding process and a two-step temperature controlling crystallization.1 Interestingly, the composites can greatly enhance a surface hydrophobicity, which significantly affects the separation capacity of CO2 from the simulated humid flue gas.1 The additional disadvantage of a conventional zeolite is the diffusion limitation of reactants and products inside microporous structures. Typically, the conventional zeolite shows a low activity in a catalytic reaction when bulky molecules are involved. This phenomenon eventually leads to the limitation of using the conventional zeolite in many industrial applications, such as hydrodeoxygenation (HDO) of lignin-derived alkylphenols as one of the most interesting bio-oil upgrading reactions25 and the conversion of hydrocarbons to aromatics26. To overcome these problems, the development of a zeolite with hierarchical porous architectures is one of the most promising strategies to improve the catalyst efficiency.27 Recently, various new hierarchical zeolites with nanosheet structures have been extensively developed because of a significant increase in inter-sheet mesopore volumes and external surface areas. 25-26, 28-31


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As stated above, zeolite or silica/MOF composites have been mainly obtained by the direct synthesis of MOFs on zeolite surfaces. This strategy sometimes suffers from the weak interactions between two materials, resulting in the phase separation.1 Recently, the scientific community has made the effort to increase their interactions by connecting MOF and silica through the chemical bond. The external surface of silica was grafted with MOF ligand moieties, which can further coordinate with the deposited MOF. For example, Sue et al. have successfully synthesized new hierarchical micro/mesoporous containing microporous zeolitic imidazolate framework-8 (ZIF-8) and siliceous mesocellular foams (MCF). The ligand of ZIF-8, 2-Methylimidazole, was grafted onto the aminefunctionalized MCF surfaces through the imine formation. The ZIF-8 particles were further formed and attached to the modified surface via the dangling 2-Methylimidazole. This strategy allows the strong interaction between two phases, reducing the phase separation of two materials.32 To integrate the beneficial effect of hierarchical zeolites and nanosheet structures together with their surface functionalization obtained via a direct chemical interaction with MOF-ligand modified surfaces, the deposition of MOFs nanoparticles on hierarchical zeolite surfaces should increase the hydrophobicity compared to a conventional one. The hydrophobic surfaces can also improve the hydrothermal stability of zeolite frameworks having low Si/Al in which they can be easily hydrolyzed in an aqueous condition33-34. In addition, the catalytic activity of the hybrid material should be improved due to the modified functional surfaces. To the best of our knowledge, the design of hierarchical zeolites/MOFs composites has not been reported so far, in particular, to demonstrate their catalytic performance in an acid-base synergistic catalysis. In the present study, we demonstrate the first example of the novel design of hierarchical FAU/ZIF-8 composites obtained by a stepwise deposition of ZIF-8 nanoparticles on amine-functionalized surfaces of zeolite nanosheets. Scheme 1 illustrates the fabrication of the composites in which the ZIF-8ligand (2-Methylimidazole) moieties have been introduced on the amine-functionalized surfaces of hierarchical FAU nanosheets via the imine formation to provide the attached ligands for constructing ZIF8. This allows designing ZIF-8 deposition on FAU surfaces via a chemical interaction of zeolite and MOF. ACS Paragon Plus Environment


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ZIF-8 nanoparticles can be formed on functionalized zeolite surfaces via the step-by-step liquid-phase approach under a mild condition in an aqueous matrix. 35-38 The degree of hydrophobicity can be easily tuned by varying the amount of deposited MOF by changing the number of deposition cycles. To illustrate their catalytic performances, the aldol condensation of acetone and 5hydroxymethylfurfural (5-HMF) or furfural (FUR) was used as the model reaction in an acid-base catalysis39. The composites compose of synergistic active sites of alkaline-exchanged metals on zeolite as basic sites and unsaturated coordinately species of ZIF-8 on the external surface and/or the crystal defects as acid-base active sites.40-42 Indeed, non-functionalized ZIF-8 surfaces are known to provide acid-base properties in which Zn(II) species and N-moieties/OH groups act as acid sites and basic centers, respectively.40-42 These catalytical properties of composites as catalysts for the acid-base catalyzed aldol condensation were also compared to the isolated zeolite and ZIF-8 as well as their physical mixtures. This first example opens up interesting perspectives for the development of new hybrid materials as acid-base catalysts for the aldol condensation of renewable biomass-derived compounds in the application of biooil upgrading.43

2. EXPERIMENTAL SECTION Materials. For the preparation of zeolites, sodium aluminate (NaAlO2: 56 wt% Al2O3 and 44 wt% Na2O, Riedel-de Haën), sodium silicate (Na2Si3O7: 26.5 wt% SiO2 and 10.6 wt% Na2O, Merck) were used as alumina and silica sources, respectively. Sodium hydroxide (NaOH: 97%, Carlo Erba) and 3(trimethoxysilyl) propyl octadecyl-dimethyl-ammonium chloride (TPOAC: 42 wt% in methanol, SigmaAldrich) were used as mineralizing agent and the structure directing agent (SDA), respectively. A 3aminopropyl-triethoxysilane (APTES: 97%, Sigma-Aldrich) and imidazole-2-carboxaldehyde (ICA: 97%, Alfa Aesar) were used as an amine functionalization and initiation linkers, respectively. For ZIF-8 synthesis, zinc nitrate hexahydrate (Zn(NO3)2•6H2O: 98%, Sigma-Aldrich) and 2-methylimidazole (mIm: 99%, Sigma-Aldrich) were used as metal center and organic linker of ZIF-8, respectively. For the catalytic


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test, a 5-hydroxymethylfurfural (5-HMF: 99%, Sigma-Aldrich), acetone (Ac: 99%, Qrec) and decane (C10H22: 99%, Sigma-Aldrich) were used as a reactant, solvent and internal standard, respectively.

Synthesis of hierarchical FAU and conventional FAU. The hierarchical FAU zeolite was synthesized following

a

modified

literature

procedure25,31

with

the

precursor

molar

ratio

of

3SiO2:3.5Na2O:1Al2O3:180H2O:0.06TPOAC. In a typical procedure, the solution A was prepared by the dissolution of NaAlO2 (4.0 g) in the NaOH solution (2.8 g). To prepare the solution B, a 14.8 g of Na2Si3O7 was added to the remaining NaOH solution. Subsequently, the solution A was slowly dropped into the solution B and vigorously stirred to obtain a homogeneous mixture before adding a desired amount of TPOAC. The reaction mixture was stirred for 24 h and placed in oven for 4 days. Finally, the obtained product was filtered, washed with excess DI water until the pH of filtrate is less than 8, and dried in oven at 110 ˚C overnight. Finally, the synthesized sample was calcined at 350 ˚C for 8 h. In order to obtain the conventional FAU, the same procedure was applied without adding any template. Surface functionalization of hierarchical and conventional FAU with aminosilane (FAU-NH2). For the functionalization of FAU surfaces by NH2-group, the FAU powder was added to the solution of 1 M of 3-aminopropyl-triethoxysilane in ethanol. The mixture was stirred for 40 h. The precipitates were then filtered, washed with excess ethanol, and dried at 110 ˚C overnight. The obtained samples are denoted as Con-FAU-NH2 and Hie-FAU-NH2 for conventional and hierarchical FAU zeolites with aminemodified surfaces, respectively. Functionalization of FAU-NH2 with imidazole-2-carboxaldehyde (ICA) (FAU-ICA). For the functionalization of FAU-NH2 with ICA, a 0.17 g of ICA was added into the mixture of FAU-NH2 in 10 ml of DMF. After aging the mixture at 130°C for 18 h, the precipitates were filtered, washed with a large amount of hot DMF to remove excess ICA, and then washed with ethyl acetate to remove DMF. Finally, the final products were dried at 100°C overnight. The obtained samples are denoted as Con-FAU-ICA and Hie-FAU-ICA for conventional and hierarchical FAU zeolites with ICA-modified surfaces, respectively. ACS Paragon Plus Environment


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Synthesis of conventional and hierarchical FAU/ZIF-8 composites via a stepwise deposition method. For the ZIF-8 deposition on FAU-ICA surfaces, a 0.37 g of zinc nitrate hexahydrate (Zn(NO3)2•6H2O) was added into the dispersed FAU-ICA in ethanol (5 ml). The resulting samples were then washed with a large amount of ethanol to remove excess Zn(NO 3)2. Subsequently, the precipitates were added into a solution of 2-methylimidazole (mIm) in ethanol (0.27 M). Finally, the first deposition cycle of ZIF-8 on FAU surfaces was obtained after washing with a large portion of ethanol to remove excess mIm. To tune the degree of ZIF-8 deposition, the deposition step was repeated until the expected number of MOF deposition cycles was reached. The obtained samples are denoted as Hie-FAU-ZIF-8-nC and Con-FAU-ZIF-8-nC for conventional and hierarchical FAU/ZIF-8 composites, respectively, and n refers to the number of deposition cycles. Synthesis of a multilayer/bulk deposition of ZIF-8 on FAU-ICA surfaces. To vary the degree of hydrophobicity of modified FAU surfaces, a multilayer or bulk deposition of ZIF-8 on FAU surfaces was also obtained for changing the amount of MOF deposit. In this case, a synthesis method was a one-pot synthesis via pH adjustment and it is different from a stepwise deposition method. The hierarchical FAUICA was added in the metal solution of 1.0x10-7 M of Zn(NO3)2. Then, the pH of the solution was adjusted to 1 by 1.0 M HCl. The prepared ligand solution (0.7x10 -5 M) was added slowly into the precursor solution. Then, the solution was stirred vigorously for 12 h at room temperature and atmospheric pressure. Finally, the composite of hierarchical FAU and multilayered ZIF-8 was successfully prepared denoted as Hie-FAU-ZIF-8-M. Catalytic testing. The solvent-free liquid-phase aldol condensation of various aldehyde reactants including 5-hydroxymethylfurfural (5-HMF) and furfural (FUR) with acetone (Ac) was performed using high pressure compact laboratory reactors (Parr reactor series 5500) equipped with PID temperature controller and magnetic drive. The reaction tests were carried out at 130 ˚C under an autogenous pressure for 6 h. Typically, a 5 ml of 10%wt of 5-HMF in acetone and a 0.25 ml of decane as an internal standard were added to a 0.25 g of catalysts. Reaction mixtures were analyzed by gas chromatography (Agilent 7820B GC) equipped with mass spectrometer (Agilent 5977A MSD) and a HP-5MS capillary column. ACS Paragon Plus Environment

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The conversion was calculated based on the consumption of aldehyde as the amount of acetone was in excess. The mass balance of all experiments is in the range of 104±5%. Characterization. Solid-state 13C CP/MAS NMR spectra were recorded by using a contact time of 3 ms performed on a Bruker AVANCE III HD-600 MHz spectrometer. Powder X-ray diffraction (XRD) patterns were collected by Bruker D8 ADVANCE instrument with 0.02˚ of step sizes in the 2θ range of 5 to 60˚. Fourier transform infrared (FTIR) spectra of samples were recorded at room temperature performed on a PerkinElmer. The average spectra were scanned sixteen times in the range of 4000−400 cm−1. The XPS depth profiles were recorded on a JEOL JPS-9010 equipped with a non-monochromatic Mg K Xrays (1,486.6 eV). An argon ion gun was used to etch the samples for 120 s. Based on an etching rate of 1 nm.s-1, XPS spectra were obtained at approximately 120 nm depth intervals. The C 1s was selected as the reference to correct the spectra, in which value was accepted equal to 284.7 eV. The atomic compositions were calculated using the relatively integrated peak areas of the Zn 2p3/2 and Al 2p3/2 orbitals as a representative of ZIF-8 and FAU, respectively. Scanning electron microscope (SEM) images were taken by using a JEOL JSM-7610F microscope with 0.5-2 kV of accelerating voltage, in gentle beam mode. The samples were mounted without crashing and Pt metal coating. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100 microscope at 200 kV. The N2 sorption experiments were performed on the Belmax apparatus at -196 °C. Prior to the measurement, the composite sample was degassed under vacuum using a two-step treatment procedure at 30 ˚C for 24 h and 120˚C for 4 h. Water contact angle (WCA) experiments were performed using a Dataphysics/OCA 20 machine. A 10 μL sized droplet was used for all measurements. Thermogravimetric analysis (TGA) was performed on a PerkinElmer/Pyris1 machine in the temperature range of 35 to 900 ˚C at 10 ˚C.min-1 under N2 atmosphere. The acid-basic properties of catalysts were determined by the temperature-programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD) and performed on a BELCAT II instrument equipped with thermal conductivity detectors (TCD). Prior to the measurement, the catalyst was preheated at 250 °C for 2 h under the flow of He. Subsequently, the catalyst was adsorbed with CO2 or NH3 at 35 ACS Paragon Plus Environment


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°C for 1 h and flushed under the flow of He for 30 min to remove the physically adsorbed CO 2 or NH3. The TPD profiles were recorded in the temperature range of 100 to 300 °C with the heating rate of 5 °C.min-1. The elemental composition of Zn2+ in a leaching solution was determined by Inductively coupled plasma–optical emission spectrometry (ICP–OES) analysis. The sample was digested in 1 . 5 ml of an acidic solution containing HF ( 48wt% in water) and HNO3 ( 65wt% in water) in the molar ratio of 1:1. Subsequently, the dissolved solution was diluted with DI water and it was determined by ICP-OES (Agilent Technologies, 700 Series) with wavelength of 2 0 2 . 5 nm. AFM (Park Systems NX10, Park Systems) was a technique to monitor the surface roughness based on three-dimensional (3D) images. The scan rate was set at 0.5 Hz and the scanning area was 5 μm × 5 μm. 1H NMR spectra were used to confirm the diastereomer of products (Figure S-10). They were performed on a Bruker AVANCE III HD-600 MHz spectrometer and referenced to protio impurity of commercial acetone-d6 ((CD3)2CO, δ 2.05 ppm) as an internal standard.

3. RESULTS AND DISCUSSION The hierarchical FAU/ZIF-8 composites were successfully prepared via a stepwise deposition method composing of the following three steps: (i) amine-functionalization on zeolite surfaces by aminosilane; (ii) imine-formation on amine-functionalized surfaces for ZIF-8 ligand attachment; (iii) stepwise deposition of ZIF-8 on functionalized surfaces. This technique can be used to increase the interaction between FAU and ZIF-8 to prevent their phase separation. To confirm the surface modification,

13

C-NMR and FTIR spectra were recorded to demonstrate the presence of their

corresponding surface modification as shown in Figures 1A and 1B, respectively. The aminefunctionalized hierarchical FAU (Hie-FAU-NH2) was prepared as the first step via the silylation of silanol surfaces of zeolites and aminosilanes. Obviously, the characteristic peaks of amine in 13C-NMR (0 to 500 ppm) and FTIR spectra (~1600 cm-1 and ~760 cm-1 for NH2 scissoring, and N-H wagging, respectively)32 confirm the presence of amine-modified surfaces.4 To attach ICA moieties on zeolite surfaces (Hie-FAUACS Paragon Plus Environment

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ICA), ICA molecules are reacted with amine groups of modified zeolites via an imine condensation as confirmed by the characteristic peaks of aromatic carbon from 13C NMR and FTIR spectra in the region of 100-150 ppm and 1200 cm-1, respectively.32 These two steps are of crucial importance to increase the interaction between MOF and zeolite by a chemical covalent bond. In the final step, ZIF-8 nanoparticles were deposited on Hie-FAU-ICA and Con-FAU-ICA via a stepwise deposition method. To verify the characteristics of deposited ZIF-8 layer on FAU surfaces, the broad peaks obtained from 13C NMR at 10, 20, 45 ppm and in the range of 100 to 150 ppm correspond to the characteristics of carbons in aminosilane ligand and carbon in ZIF-8 ligand, respectively.32 Moreover, the intensity of FTIR characteristic peaks of deposited ZIF-8 (Hie-FAU-ZIF-8) appeared at 1375-1500 cm-1 is more obvious than that of the Hie-FAUICA because there is much more content of aromatic rings as organic linkers of ZIF-8. Interestingly, the FTIR characteristic peaks of FAU at ~530 and ~430 cm-1, representing six membered rings of FAU, are also obtained in the case of composites.44



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Figure 1. (A) 13C NMR, (B) FTIR spectra, (C) XRD patterns and (D) relationship of Zn 2p3/2 and Al 2p3/2 concentration as a function of etching time (depth from surface) in Hie-FAU-ZIF-8-15C obtained from XPS depth profile analysis. The obtained samples are denoted as a) Hie-FAU-ZIF-8-10C, b) HieFAU-ICA, c) Hie-FAU-NH2, d) Hie-FAU, e) ZIF-8 and f) Con-FAU-ZIF-8-10C. To further confirm the crystalline structure of zeolite/MOF composites, the structure of assynthesized conventional and hierarchical FAU/ZIF-8 composites obtained by ten cycles of MOF nanoparticles deposition (Con-FAU-ZIF-8-10C and Hie-FAU-ZIF-8-10C) were characterized by X-ray powder diffraction (XRD) as shown in Figure 1C. Compared with unmodified FAU samples, the diffraction pattern of the composite shows implicitly characteristic peaks at 2θ of 6.10, 9.97, 11.69 and 15.39 ˚ corresponding to the individual character of FAU.25 In addition, the characteristic crystalline peaks of ZIF-8 appear at 2θ of 7.3, 12.7 and 18.0 ˚ corresponding to (110), (211) and (222) crystal planes, respectively45, confirming the presence of ZIF-8 in the composite. Interestingly, the characteristic XRD peaks of ZIF-8 are more pronounced when increasing the number of deposition cycles (Figure S-1). In addition, the FAU surface modification by ZIF-8 was monitored by the XPS depth profile analysis of Zn (Figure S-2A) and Al (Figure S-2B). In the depth profile of the composite, the intensity of Al (2p3/2) peak obviously increases, whereas the Zn (2p3/2) peak intensity gradually decreases as a function of etching time (Figures 1D and S-2). The etching time relates to the depth from the surface. The decrease of Zn (2p3/2) atomic percentage (%) as a function of etching time indicates that the portion of ZIF-8 is dominant at the outermost surface of the composite, whereas FAU is dominant inside the structure. These observations explicitly confirm again that the surface modification of FAU by ZIF-8 has been successfully prepared via the stepwise deposition method.


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Figure 2. Surface morphologies of hierarchical FAU/ZIF-8 composite obtained by SEM images: (A) Hie-FAU; (B) Hie-FAU-ZIF-8-10C, and TEM image: (C) Hie-FAU-ZIF-8-10C, and surface morphologies of conventional FAU/ZIF-8 composite obtained by SEM images: (D) Con-FAU; (E) ConFAU-ZIF-8-10C, and TEM image: (F) Hie-FAU-ZIF-8-10C.

Morphologies of hierarchical FAU/ZIF-8 (Hie-FAU-ZIF-8-10C) and conventional FAU/ZIF-8 composites (Con-FAU-ZIF-8-10C) were examined by SEM and TEM images (Figure 2). The assembled particles of isolated hierarchical FAU nanosheets are in the size range of 3.6±0.4 µm, whereas the sizes of conventional zeolite crystals are in the range of 2.0±0.3 µm that is similar to what has been described in previous reports.25,31 Interestingly, different morphologies of unmodified FAU and composites are clearly observed. The change of morphologies from smooth-surface crystals to rough surfaces implies that ZIF-8 nanoparticles can be deposited on external surfaces of both hierarchical FAU (Figures 2B-2C) and conventional FAU (Figures 2E-2F). To confirm the change of the surface textures of the composite, AFM image of the composite reveals that the Root Mean Square (RMS) roughness of the composite increases from 4.44 to 97.13 nm compared to bare hierarchical FAU as shown in Figure S-3A. This increase in the RMS roughness strongly confirms the presence of nanoparticles deposited on hierarchical FAU surfaces. As stated above and also illustrated in Figure S-1, the number of deposition cycles also affects the intensity of XRD characteristic of ZIF-8. This eventually leads to the formation of different ACS Paragon Plus Environment


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size of ZIF-8 nanoparticles (Figure S-3B). For example, the size of ZIF-8 nanoparticles deposited on hierarchical FAU obtained by ten and fifteen deposition cycles is approximately 57±7 and 70±8 nm, respectively. In addition, the size of ZIF-8 nanoparticles deposited on hierarchical FAU is slightly larger than that of the conventional one (Figure S-3B). To compare the amount of ZIF-8 in composites obtained by different deposition cycles, the weight loss in the composite obtained from TGA profiles relates to the MOF content and it strongly depends on the number of deposition cycles (Figures 3A and S-4). Obviously, the ZIF-8 content in the hierarchical composite obtained from the weight loss in the temperature range of 300-600 ˚C referred to the complete thermal decomposition of ZIF-8 to ZnO increases from 10 to 30 wt.%, when changing deposition cycles from one to fifteen.46 In addition, to confirm the content of Zn in the composite, the digested sample was analyzed by ICP-OES. For example, the Hie-FAU-ZIF-8-15C contains 13.85 wt. % of Zn2+, which corresponds to the mole ratio of Zn to the organic linker of 1:2. These results again confirm the stoichiometric ratio of Zn and linker of ZIF-8 in the composite. As expected, the mass content of ZIF-8 dramatically increases up to 48 %, when depositing using a multilayer or bulk deposition method as shown in Figure S-4. This content corresponds to the 90 % of ZIF-8 yield produced using an aqueous synthesis.46 Compared to the conventional FAU, an increase of external surface area of FAU nanosheet as shown in Table S-1, implying the increasing number of silanol surfaces, can greatly enhance the degree of MOF deposition. This behavior leads to increase in the amine-grafted surface density from 6 to 8 molecules.nm2

(obtained from CHN results) for Con-FAU-NH2 and Hie-FAU-NH2, respectively, resulting in enhancing

the deposited MOF content (Figure 3A). Typically, the organic grafting or nanoparticles deposition on zeolite surfaces strongly affects the porous structure of zeolite (Table S-1). Unsurprisingly, the surface area of composites and aminefunctionalized zeolites 47 was significantly reduced compared to the isolated Hie-FAU. This phenomenon could also be found in many cases when using the silane agents for zeolite surface modification47-49. Therefore, the reaction of silane groups or amine grafted molecules and the hydroxylated surfaces occurred at the pore channels can block the accessibility of guest molecules, and these moieties can be ACS Paragon Plus Environment

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further attached to the organic linkers to produce ZIF-8 at the zeolite pore mouth, resulting in a decrease of a specific surface area. However, an increase in a specific surface area of FAU/ZIF-8 composite can be observed when increasing deposition cycles (Table S-1). This might be explained by the enhancement of the microporous feature of ZIF-8 as a function of the deposited content (Table S-1).

Figure 3. (A) TGA results of Con-FAU/ZIF-8 and Hie-FAU/ZIF-8 composites obtained by various deposition cycles (1, 5, 10, and 15 cycles) and (B) water contact angle (WCA) results taken from 15 seconds of water droplet freezing time of FAU, Hie-FAU/ZIF-8 composites obtained by various deposition cycles (1, 5, 10, and 15 cycles) and isolated ZIF-8. As stated above, it should be possible to increase the interaction between MOFs and zeolite surfaces by the covalent bonding of MOF ligands and silanol surfaces. To further confirm this hypothesis, the composites of hierarchical FAU and multilayered ZIF-8 (Hie-FAU-ZIF-8-M) were synthesized via a multilayer or bulk deposition method, which can provide a higher content of MOF deposition via pH adjustment. As expected, the MOF ligand-functionalized zeolite surface (Hie-FAU-ICA) prevents the phase separation of two materials, zeolite and MOF (Figure S-5A). In strong contrast to this, the presence of isolated ZIF-8 and FAU crystals resulting from the deposition of MOF on unmodified zeolite surfaces was clearly observed (Figure S-5B). This is consistent with the view that the surface modification before



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grafting of MOF nanoparticles on hydroxyl surfaces is of a crucial step to prevent the material phase separation. To illustrate the improvement of hydrophobic surface properties of composite surfaces belonging to ZIF-8 characteristic, the water contact angle (WCA) measurement was investigated as shown in Figure 3B and Figure S-6 for the static WCA and the time sequence of water droplets on different samples, respectively. The WCA, relating to the wettability of surface of the composites, dramatically increases compared with those of the bare FAU. It is clearly seen that the significant increase of freezing time for water droplets over the composite relates to the improvement of hydrophobic surfaces (Figure S-6). In addition, the WCA of hierarchical FAU/ZIF-8 composite increases as a function of deposited ZIF-8 content and it exhibits the plateau WCA at 15-70° for the sample obtained by fifteen deposition cycles. Compared to the hierarchical FAU/ZIF-8, the significantly lower WCA can be observed in the case of the conventional composite (Figure 3B and Figure S-6). This is most likely due to the fact that the high silanol surface density can greatly enhance the degree of surface functionalization. These observations confirm again that it has been successfully achieved to improve the hydrophobic surface functionalization using a hierarchical zeolite structure. The acid-basic properties of the hierarchical FAU/ZIF-8 composite were investigated by NH3 and CO2 TPD techniques. To compare the acidity of the composite and isolated ZIF-8, the NH3-TPD curve obviously demonstrates the presence of acidic surfaces on the FAU/ZIF-8 composite as shown in Figure S-3C(a) and Table S-2. It was found that the composite can greatly enhance the acidic properties compared to the isolated ZIF-8. In addition, the basicity of the composite was also investigated by CO 2-TPD as shown in Figure S-3C(b) and Table S-2. It clearly shows that the higher basicity can be found in case of the composite compared to the bare FAU and ZIF-8. These observations are very convincible that the FAU/ZIF-8 composite can improve both acid and basic properties, which should also be possible to enhance the catalytic performance in the acid-base catalysis.


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Figure 4. Reaction results of Hie-FAU, Con-FAU, Hie-FAU-ZIF-8- nC composites in 1, 5, 10, 15 deposited layers, Hie-FAU-ZIF-8-M and ZIF-8 using 5-HMF as a reactant. In order to test the described concept using zeolites with MOF-modified surfaces as highly efficient acid-base catalysts, the aldol condensation of various aldehyde reactants such as 5hydroxymethylfurfural (5-HMF) and furfural (FUR) with acetone (Ac) was used as the acid/basecatalyzed model reaction. Recently, the significantly improved catalytic activity of the aldol condensation can be observed when using the hierarchical FAU nanosheets.47 However, to increase their catalytic activity as acid or base catalysts the further surface modification is required.47 In this work, the hierarchical FAU/ZIF-8 composite can dramatically enhance the catalytic activity of the aldol condensation of 5-HMF and acetone to selectively produce the 4-[5-(hydroxymethyl)furan-2-yl]but-3-en2-one (HMB) as a product (Figure 4 and Table S-2). For example, the yield of HMB increases from 16 to 40 % when using the composite obtained by five deposition cycles of ZIF-8 (Hie-FAU-ZIF-8-5C) compared to the bare hierarchical FAU and the organic amine-grafted hierarchical FAU under the same catalytic condition. In contrast, the physical mixture of hierarchical FAU and ZIF-8 in the same weight ratio as Hie-FAU-ZIF-8-5C shows the low activity of HMB production (HMB yield of 16%). The reason for this increase in activity of the composite relates to the fact that the basic properties of the alkaline exchange FAU together with unsaturated coordinately ZIF-8 structures; bearing Zn(II) species as acid sites and N-moieties as basic actives, play simultaneously an important role in the synergistic effects ACS Paragon Plus Environment


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between the facile activation of acetone by base and acid active species to form enolate19 and enol anions, respectively, and the stabilization of carbonyl species by zeolite framework, facilitating a nucleophilic attack in the further step of reaction50 (see reaction mechanisms on both acid and base active sites in Figure S-7). Interestingly, the degree of catalytic activity of the composite can be easily tuned by changing the deposited ZIF-8 content. It was found that the HMB yield can be approximately 67 % when using the composite obtained by fifteen deposition cycles (Hie-FAU-ZIF-8-15C). However, the yield of a product starts to decrease when using a too high amount of ZIF-8 that can be obtained in the case of a bulk deposition method (Hie-FAU-ZIF-8-M). The reason for this decrease in activity might be explained by the diffusion limitation of bulk MOF crystals (Figure S-5) hindering the accessibility of guest molecules to active sites of FAU. Therefore, the global activity only comes from the catalyzed ZIF-8 as active species. To confirm this hypothesis, the isolated ZIF-8 was applied under the same condition (Figure 4). The HMB yield over ZIF-8 is, as expected, close to those of the Hie-FAU-ZIF-8-M. This makes it clear that the bulk deposition method only produces a pure MOF-like character, resulting in the low catalytic performance as same as the isolated system. These observations confirm the benefit of a stepwise deposition technique providing a highly efficient acid-base catalyst. In order to generalize the use of composites for the aldol condensation with various reactants, we carried out an additional experiment for the aldol condensation of furfural (FUR) and acetone (Table S2). A similar tendency can also be observed by the increase in the activity when using the composite obtained by a stepwise deposition method (Hie-FAU-ZIF-8-15C) compared to the bare hierarchical FAU (26, 15 and 2 % yield of product obtained by using Hie-FAU-ZIF-8-15C, bare Hie-FAU and isolated ZIF8, respectively). It also confirms again that the catalytic activity significantly decreases when using the Hie-FAU-ZIF-8-M obtained by a bulk deposition method. In addition, in the aldol condensation of aliphatic aldehyde, acetaldehyde, with acetone, it was found that the composite can also catalyze the reaction with the selectivity of 3-penten-2-one, the aldol condensation product, of 40% at the 56% of the


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conversion level at a short reaction time (6 h). These observations clearly show that various aldehydes can be used as reactants for the aldol condensation using the hierarchical FAU/ZIF-8 composite. To test the reusability of catalysts, the used composites were recovered from the reaction mixture and repeatedly dispersed in a new reactant solution. Interestingly, the results, as shown in Figure S-8 exhibit very promising activity even after three catalytic cycles of reusability test. The percentage of conversion of 5-HMF after three cycles is almost the same (~60%) and the high selectivity of HMB is still observed (~98%) with a very small contaminant of 4-hydroxy-4-methyl-2-pentanone (HMP) as a self-condensation product (< 2%). The leaching of active sites was also examined by ICP-OES technique in order to obtain the removal of ZIF-8 component from the composite during the reaction. The calibration curve of the concentration of Zn2+ as the function of the intensity is shown in Figure S-8B. Obviously, only a very small content of Zn2+ (< 0.5 wt.%) was detected (Figure S-8A) even after three repeated catalytic cycles, implying that no significant active site fraction was leached from the composite during the catalytic reaction. In conclusion of the catalytic performance of catalysts, the composite exhibits a very highly chemically stable in terms of reusability providing high efficiency in the aldol condensation reaction. In addition, the morphology and crystalline structure of the spent catalyst were also investigated (Figure S9). The composite still maintains the framework structure of FAU and ZIF-8 with a slight decrease in the relative crystallinity approximately 29%. It clearly shows that the modest stability of the composite can be observed even after several cycles of the catalytic reaction. These observations clearly demonstrate that the hierarchical FAU nanosheets can improve the degree of surface functionalization of ZIF-8 on zeolite surfaces. This behavior eventually produces the excellent hybrid hierarchical FAU/ZIF-8 composites, in particular, when applying with a controllable stepwise MOF deposition method. The novel designed composites have played an important role in the term of synergistic effects between basic site environments of Na+ stabilized zeolite framework and unsaturated ZIF-8 structure bearing Zn(II) species as acid sites together with N/OH-moieties as basic actives37-39 for the acid-base-catalyzed aldol condensation. ACS Paragon Plus Environment


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4. CONCLUSION The hierarchical FAU/ZIF-8 composites have been successfully prepared by a stepwise deposition of ZIF-8 on imidazolate linker-modified zeolite surfaces. Obviously, the degree of surface functionalization can greatly enhance due to the presence of hierarchical structures. This behavior leads to increase in the MOF deposited content, resulting in the improvement of the hydrophobic surface properties of zeolite surfaces. In addition, the MOF content can be easily tuned by changing the number of deposition cycles. Interestingly, the novel designed hierarchical zeolite/MOF composite exhibits outstanding catalytic properties as an acid-base catalyst for the aldol condensation of various aldehydes such as 5HMF and furfural (FUR). Compared to the isolated FAU and ZIF-8, the high yield of 4-[5(hydroxymethyl)furan-2-yl]but-3-en-2-one (HMB) (67 %) can be observed over the composite obtained by a stepwise deposition method due to the synergistic effect between Na + stabilized zeolite framework and ZIF-8 catalytic functions. This opens up interesting perspectives for the development of new organic and inorganic hybrid materials as heterogeneous acid-base catalysts.

ASSOCIATED CONTENT Supporting Information Additional XRD patterns, XPS depth analysis, AFM, bar chart of particle sizes, TPD, TGA of multilayer composites, SEM images of multilayer composites, water contact angle (WCA) examples, proposed reaction mechanisms of aldol condensation, bar chart of reusability of catalytic performances and leaching results, ICP calibration curve of Zn2+, SEM images and XRD patterns of spent catalyst after reusability tests, 1H NMR spectra of reaction mixture, table of textural properties, table of concentration of acid and basic sites and table of catalytic performance results. AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Author * E-mail: [email protected]. Present Addresses †

Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering,

Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand Author Contributions D.S. and C.W. conceived and designed the experiments; D.S., W.W., and T.Y. performed the experiments; S.I., T.U., and K.P. contributed in characterization techniques; D.S., S.B., and C.W. co-produced the manuscript. ACKNOWLEDGMENT This work was supported by the research grant from Vidyasirimedhi Institute of Science and Technology (VISTEC), the Thailand Research Fund (TRF) (MRG6180099), the Office of Higher Education Commission (OHEC), and the Junior Research Fellowship Program of the French Embassy in Thailand. Furthermore, the authors would like to acknowledge the Frontier Research Center (FRC), VISTEC, and NANOTEC Center of Excellence on Nanoscale Materials Design for Green Nanotechnology at Kasetsart University for technical support. This work has also benefited from the facilities and expertise of the characterization techniques from Prof. Dr. Jumras Limtrakul, Ms. Pimpisut Worakajit and Ms. Sopanat Sawatdee. ABBREVIATIONS MOF, Metal Organic Framework; ZIF-8, Zeolitic Imidazole Framework-8; Hie-FAU, Hierarchical Faujasite zeolite; Con-FAU, Conventional Faujasite zeolite; Hie-FAU-NH2, Hierarchical Faujasite zeolite with amine functionalization; Hie-FAU-ICA, Hierarchical Faujasite zeolite with MOF’s ligand functionalization; Hie-FAU-ZIF-8-nC, Hierarchical Faujasite zeolite/ZIF-8 composite with n deposited ACS Paragon Plus Environment


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cycles (n = 1, 5, 10 and 15); 5-HMF, 5-hydroxymethylfurfural; FUR, Furfural; HMB, 4-[5(hydroxymethyl)furan-2-yl]but-3-en-2-one; HMB, 4-[5-(hydroxymethyl)furan-2-yl]but-3-en-2-one. REFERENCES 1.

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catalysts for the Mukaiyama aldol reaction: A DFT mechanistic study. ChemPhysChem 2013, 14, 923-928.


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ZIF-8 Hybrid Materials as Highly ... - ACS Publications

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